Syncope, or fainting, is a transient loss of consciousness and postural tone that may be due a number of etiologies, both cardiovascular and non-cardiovascular. The most common pathophysiogical basis of syncope is an acute decrease in cerebral blood flow secondary to a decrease in cardiac output, thereby causing cerebral hypoxia. Such a decrease in cardiac output can be due to, for example, cardiac arrhythmias or cardiac outflow obstructions. Neurocardiogenic syncope is a relatively benign condition in which dysfunction of the autonomic nervous system causes an inappropriate slowing of the heart (bradycardia) to result in hypotension. Classic neurogenic syncope (vasovagal syncope) occurs when inappropriate reflex inhibition of the sympathetic nervous system and increased parasympathetic activity causes both bradycardia and peripheral vasodilation. Vasovagal syncope may occur in otherwise healthy individuals and in patients with a variety of underlying diseases. A number of factors may precipitate vasovagal syncope, including a hot or crowded environment, alcohol, extreme fatigue, hunger, chronic recumbency, prolonged standing, and emotional or stressful situations. Another type of neurocardiogenic syncope involves failure of the baroreceptor reflex to transiently increase the heart rate when an individual rises to an upright position, causing venous pooling in the lower extremities and decreased venous return to the right side of the heart.
Even if the cause of the syncope is benign, however, its consequences may not be. Falls during syncope can result in fractures, and episodes that occur while driving can be extremely dangerous. Chronic and recurring syncope can create a level of functional impairment similar to that produced by other chronic debilitating disorders.